1. Field of the Invention
The present invention relates to an apparatus and method for adjusting the level of a nuclear fuel rod in a nuclear reactor fuel assembly.
2. Description of the Prior Art
Conventional fuel assemblies of the type used in commercial nuclear reactors for generating electric power typically comprise a plurality of fuel rods, with each rod including a stack of fuel elements mounted inside a metal tube which is sealed by end plugs. The fuel rods are held in a laterally spaced apart generally rectangular arrangement by a supporting "skeleton" which consists of a number of grids which are spaced at lengthwise intervals along the length of the assembly. The fuel rods are individually mounted to the grids by means of spring mountings, which permit limited axial movement of each rod. Control rod guide thimbles extend parallel to the rods and serve to interconnect the grids, and the guide thimbles extend axially beyond the ends of the rods for mounting top and bottom nozzles.
It is important for the proper functioning of the fuel rods that all of the rods be positioned in the assembly at the same level, and so that the ends of all of the rods lie in the same plane at each end of the assembly. Thus each end of each rod should be spaced from the adjacent nozzle a predetermined gap distance. However, when the fuel rods are axially loaded into the assembly, it is difficult to insure that all rods are positioned at the proper level, and it is common for certain ones of the rods to be loaded at an improper level. When this occurs, the nozzle at one end of the assembly must be disassembled so as to permit the misaligned fuel rods to be manipulated and the gap distance to be corrected. As will be apparent, the required disassembly and subsequent reassembly of the nozzle is a time consuming and thus expensive operation.
The prior U.S. patent to Doss et al, No. 4,265,010, discloses a tool for adjusting the level of a fuel rod in a fuel assembly, and which is designed to avoid the necessity of disassembling the nozzle to correct the level of misaligned rods. In particular, the tool includes one end which is insertable between the nozzle and the exposed ends of the fuel rods, and the insertable end includes two plates which may be laterally separated by actuating a threaded interconnection between the plates, to thereby move the misaligned fuel rod with respect to the nozzle. While this tool represents a significant improvement in the art, the tool must be carefully used to insure that the final spacing is correct, and also to insure that no lateral forces are imparted to the rod which could adversely effect its lateral separation from the adjacent rods in the assembly.
It is accordingly an object of the present invention to provide an apparatus and method for adjusting the elevation of a fuel rod in a nuclear reactor fuel assembly, and which is adapted to efficiently adjust the level of individual rods without requiring the disassembly of an end nozzle.
It is also an object of the present invention to provide an apparatus and method of the described type which is adapted to adjust the gap between a misaligned rod and the nozzle to a predetermined maximum distance, which corresponds to the proper gap distance.
It is still another object of the present invention to provide an apparatus and method of the described type which does not exert an unacceptable lateral force on the fuel rod during axial movement thereof.